Platinum Contacts in Automatic Landing Systems

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All BEA Trident aircraft are equipped with the Smiths’ Series 5 flight control system. This “blind landing” system has steadily increased in application since its first use in June 1965. At the present time 100 or more automatic landings using this system are made by Trident aircraft every month. This demonstrates the very real lead in fully automatic operations which has been achieved by wholly British technology.

The circuitry of this flight control system incorporates some 2,000 micro relays of the single-pole changeover type which are hermetically sealed. The 10 per cent rhodium-platinum contacts in each relay are gold-plated and are suitable for switching either very low signal energies or higher powers up to 15 VA. The relay is designed with a strong accent on reliability which is clearly of prime importance in this application.

The 10 per cent rhodium-platinum is used in order to guarantee a contact resistance of less than 0.1 ohm. The alloy is in the form of 0.017 inch diameter wire which is gold-plated to a depth of 150 micro-inches. Some three inches of this wire is used in each relay.

The relay is sealed by passing the rhodium-platinum wires through a glass pellet and fusing the glass on to the rhodium-platinum at 1000°C. This glass-to-metal sealing work is carried out on Smiths’ behalf by Wesley Coe Ltd, of Cambridge.